In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. For example, the ink can be first transferred to an intermediate blanket that in turn is used to transfer the ink to the surface of the material upon which the image is to be reproduced.
Recent developments in the field of printing plate precursors concern the use of lasers or laser diodes for imaging. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
Imageable elements useful to prepare lithographic printing plates typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components that can be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the non-imaged regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the element is considered as positive-working. If the non-imaged regions are removed, the element is considered as negative-working. In each instance, the regions of the imageable layer (that is, the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions (typically a fountain solution) and repel ink.
Imaging of the imageable element with ultraviolet and/or visible radiation is typically carried out through a mask that has clear and opaque regions. Imaging takes place in the regions under the clear regions of the mask but does not occur in the regions under the opaque mask regions. Use of a mask is time-consuming and has a number of significant disadvantages.
Direct digital imaging has obviated the need for imaging through a mask and is becoming increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. Thermally imageable, multi-layer elements are described, for example, U.S. Pat. No. 6,294,311 (Shimazu et al.), U.S. Pat. No. 6,352,812 (Shimazu et al.), U.S. Pat. No. 6,593,055 (Shimazu et al.), U.S. Pat. No. 6,352,811 (Patel et al.), U.S. Pat. No. 6,358,669 (Savariar-Hauck et al.), and U.S. Pat. No. 6,528,228 (Savariar-Hauck et al.), U.S. Patent Application Publication 2004/0067432 A1 (Kitson et al.).
Radiation-sensitive compositions and imageable elements have also been designed for imagewise exposure and/or development while on a printing press using fountain solutions and/or printing inks, as described for example in U.S. Pat. No. 6,582,882 (Pappas et al.) and U.S. Pat. No. 6,899,994 (Huang et al.), U.S. Patent Application Publication 2004/0260050 (Munnelly et al.), and WO 2004/101280 (Munnelly et al.). These imageable elements typically comprise a single imageable layer on the substrate.
U.S. Pat. No. 6,143,464 (Kawauchi) describes positive-working photosensitive compositions containing a blend of phenolic resins and a copolymer derived from acrylic monomers with pendant sulfonamide groups. These compositions are used to prepare single-layer imageable elements.
Multilayer imageable elements containing sulfonamide polymers are described in U.S. Pat. No. 6,352,811 (Patel et al.) and U.S. Patent Application Publication 2002/0081522 (Miyake et al.).
Copending and commonly assigned U.S. Ser. No. 11/337,778 (filed Jan. 23, 2006 by Saraiya, Kitson, Mikell, and Novoselova) describes multi-layer positive-working elements containing a resin with pendant sulfonamide groups in outer ink-receptive layers to reduce the formation of sludge in seasoned developer solutions.